by Paul Gilster | Jan 6, 2009 | Exoplanetary Science |
If, as we have often speculated in these pages, there is a brown dwarf closer to us than the Centauri stars, it may well be the WISE mission that finds it. The Wide-field Infrared Survey Explorer is a 40 cm telescope cooled below 17 K (-430 Fahrenheit) that will image the entire sky in four infrared wavelengths. If we’re looking for nearby brown dwarfs, an all-sky survey like this is the way to go, because such stars should be distributed uniformly in the space around us.
According to information Amanda Mainzer (JPL) presented yesterday at the American Astronomical Society meeting in Long Beach (CA), brown dwarfs are now thought to make up two-thirds of the stars in our stellar neighborhood, most of them as yet undetected. One of them might well be closer than the 4.3 light years that separate us from Alpha Centauri. And WISE should be up to the challenge of finding it, being able to detect cool brown dwarfs (down to 200 K) at Centauri distance and objects down to Jupiter-mass if closer than one light year.
We’re at the bridge between planets and stars here. L and T-class brown dwarfs include objects with temperatures down to 600 K (620 F), but WISE should be able to find numerous brown dwarfs that are cooler still, objects that may lead us to define a new spectral class. Current estimates are that the mission will uncover up to a thousand new brown dwarfs — we’ll see how close some of them may be when the mission goes to work after a launch scheduled for November of this year.
More from the brown dwarf front: The Hubble Space Telescope has been used in a study of 233 multiple star systems, which turned up only two brown dwarfs as companions to normal stars. Sergio Dieterich (Georgia State University) reported on this one yesterday at AAS, noting that the lack of brown dwarfs around solar-type stars seems to be telling us something:
“We still did not find brown dwarfs around small red stars whose mass is only slightly above the hydrogen burning limit. Especially when we consider the fact that brown dwarfs binaries do exist, the fact that there are very few binaries whose components lie on different sides of the hydrogen burning limit is significant.”
This work is part of the RECONS (Research Consortium on Nearby Stars) survey, which looks for the Sun’s neighbors within 10 parsecs (32.6 light years) of Earth. The current total of brown dwarfs in this range of space is twelve, compared with 239 identified red dwarf stars. Now the Hubble work shows that brown dwarfs don’t co-exist readily even with these small stars, an indication that mass is not the operative element in keeping these stellar types apart. “If mass ratio was the driving factor,” says Dietrich, “we would expect to find more brown dwarfs around small red stars than around solar type stars.”
AAS will also be the scene of another brown dwarf report, this one by Micaela Stumpf (Max-Planck-Institute for Astronomy, Heidelberg), which likewise implies that brown dwarfs do not tend to be found in the company of larger stars. Stumpf also reports that the brown dwarf binary Kelu-1 AB may actually be a triple system, accounting for the puzzling discrepancy between the masses of the two known dwarfs and the total mass of the system, which had been estimated on the basis of orbital dynamics. If confirmed, Kelu-1 AB may turn out to be the first known triple brown dwarf system. We have high hopes for WISE, but the deep infrared searches of the next decade may be what it takes to unravel the true size and character of the brown dwarf population.
On Kelu-1 AB, see Stumpf, “Kelu-1 AB – A possible brown dwarf triple system,” submitted to Astronomy & Astrophysics and available online.
by Paul Gilster | Jan 5, 2009 | Exoplanetary Science |
The American Astronomical Society meeting now in session in Long Beach (CA) is already making news. Led by Michael Jura (UCLA), a team of scientists has used Spitzer Space Telescope data to study six white dwarf stars that are surrounded with the remains of asteroids. The assumption here is that these materials are a likely indication of planetary formation in these systems, for they’re the same materials that go into making up the Earth and other rocky worlds in our own Solar System.
“If you ground up our asteroids and rocky planets, you would get the same type of dust we are seeing in these star systems,” says Jura, who presented the results at the meeting this morning. “This tells us that the stars have asteroids like ours — and therefore could also have rocky planets.”
When a star like our Sun reaches the end of its life and becomes a red giant, it consumes any inner planets and perturbs the orbits of the surviving planets and asteroids. A white dwarf is the end result of this stellar expansion and subsequent collapse. Objects wrenched out of their former orbits should, like the asteroids in question, occasionally drift close enough to the star to be pulled apart by its gravity. Such a star, showing the excess infrared signature of a circumstellar disk that is likely caused by the tidal disruption of asteroids, is called a ‘polluted’ white dwarf, a total of eight of which have now been studied in detail.
All eight of these stars are thought to show the results of asteroid breakup within the last million years. This is a comparatively new study, one enabled by the fact that asteroids destroyed in this way are broken into tiny pieces that are distinctive compared to asteroid dust around younger stars. The dust is subject to study with Spitzer’s infrared spectrograph, which shows a glassy silicate mineral similar to olivine, commonly found on Earth. “This is one clue,” says Jura, “that the rocky material around these stars has evolved very much like our own.”
Image: NASA’s Spitzer Space Telescope set its infrared eyes upon the dusty remains of shredded asteroids around several dead stars. This artist’s concept illustrates one such dead star, or “white dwarf,” surrounded by the bits and pieces of a disintegrating asteroid. These observations help astronomers better understand what rocky planets are made of around other stars. Credit: NASA/JPL-Caltech.
Also provocative is the lack of carbon in this debris, similar to the low carbon values found in the asteroids and rocky planets of our Solar System. From the paper on this work:
The rocky material in the inner solar system is carbon deficient. In CI chondrites – the most primitive of meteorites – and in Earth’s mantle, n(C)/n(Fe) is 0.9 and 0.009, respectively… Our new data are consistent with the view that extrasolar asteroids are carbon-deficient by a factor of 10 or more… Therefore the building blocks of extrasolar rocky planets have the same substantial carbon deficiency as found in the inner solar system.
Studying the elements present in asteroid dust in visible light should offer up still more information about their relative abundance. Polluted white dwarfs thus offer a window of sorts into how planetary materials are processed around other stars, one that tells us that rocky planet formation should not be unusual elsewhere if in fact materials like these are common in the universe.
The paper is Jura et al., “Six White Dwarfs with Circumstellar Silicates,” in press at the Astronomical Journal and available online.
by Paul Gilster | Jan 3, 2009 | Culture and Society |
What do you get out of science fiction? We’d all answer that question differently, I suppose, and surely the breadth of concepts and startling ideas is at the top of the list. But for me, the real beauty of the form is landscapes. I sometimes find myself reading a paragraph and then just putting the book down to mull over what I’ve just ‘seen.’ As in this passage from Jack McDevitt’s 2004 novel Polaris. Here, Jack is describing Sacracour, the inhabited moon of the gas giant Gobulus, which orbits its star at a distance of 160 million kilometers:
Most of the planet’s contemporary inhabitants — there are fewer than three hundred thousand altogether — live along a seacoast that’s usually warm and invigorating. Lots of beach and sun. Great sky views. They haven’t yet achieved tidal lock, so if you time things right you can sit out on the beach and watch Gobulus, with its rings and its system of moons, rise out of the ocean.
Small descriptions like that dazzle me, the off-hand observation that brings home the staggering variety of planetary settings we’re likely to encounter. I could fill a book with passages from science fiction stories that create such moments, but this is the one I had at hand yesterday when the feeling hit me again. And the more I think about it, the more I realize that feeling goes all the way back to childhood, where it emerged from writers like Heinlein, Clarke and Andre Norton.
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The first Carnival of Space of the new year is now available via Cheap Astronomy. Those with a deep space interest will want to see the post on Triton on the Supernova Condensate site, where the intriguing moon is examined in terms of its tenuous atmosphere and its surprising geysers, some of which Voyager 2 observed to be rising fully eight kilometers, the liquid nitrogen analog to Yellowstone National Park. But it’s that atmosphere and its properties that gets my attention:
Nitrogen ices encrust the planet’s surface, with some evaporating to form a thin nitrogen atmosphere. Surprisingly, there’s a little more in common with Earth here than you might realise. Specifically, Triton’s atmosphere has a troposphere — a region with weather. Rising a mere 8 km high, this region is thought to have prevailing seasonal winds. Clouds of nitrogen ice particles form here, and a haze of nitrogen rich hydrocarbons (such as nitriles) has also been found, alongside clouds of condensing nitrogen gas lower down. This starts to paint a nice little picture of nitrogen snow falling amid the gassy plumes. Triton could be said to have a nitrogen cycle, with a tenuous parallel to the cycle of water on Earth. Above the troposphere, the rest of Triton’s atmosphere extends for another 800 km above the surface, forming a neatly structured thermosphere, ionosphere and exosphere.
With a surface that is apparently the scene of extensive slush flows from cryovolcanism (consider how young the surface looks, with few impact craters on display in the areas Voyager could see), Triton’s ‘cantaloupe’ terrain could keep geologists occupied for years once we get out there for a closer look. Remember that Voyager was able to image only about forty percent of the surface and you realize how much there remains to be discovered on this enigmatic object.
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And speaking of astronomy blogs, if you’re looking for the ultimate list, check out the Top 100 Space and Astronomy Blogs recently published on the Find Schools Online site. We all try to keep up with the constant activity of bloggers around the globe, but new talent emerges quickly and it’s all too easy to keep revisiting the same places without looking around at what’s new. I find many old friends here but quite a few newcomers whose work will surely wind up in my RSS feeds. A blog devoted to Spaceports? Why not, or how about the Space Elevator Blog? The beauty of the format is that we can tackle a single topic with passion, as the many fine bloggers represented on this list continue to do.
by Paul Gilster | Jan 2, 2009 | Asteroid and Comet Deflection |
The 1300-year cold spell known as the Younger Dryas is back in the news. The sudden climate change, occurring between twelve and thirteen thousand years ago, may be related to the extinction of large species like the saber-tooth tiger and could have something to do with the disappearance of the Clovis culture, a people whose arrival in the Americas can be traced through their distinctive artifacts. Last year a team from sixteen institutions proposed that the climate change was the result of an impact event possibly involving multiple airbursts of cosmic debris.
That theory has been regarded with skepticism, but Douglas Kennett (University of Oregon), who worked with the original team, now says that its research has uncovered billions of nanometer-sized diamonds concentrated in sediments in six locations, ranging from Arizona to Oklahoma, Michigan, South Carolina, Manitoba and Alberta. Such nano-diamonds are produced under the kind of high temperatures and pressures associated with impacts and have been found in meteorites. Says Kennett:
“The nanodiamonds that we found at all six locations exist only in sediments associated with the Younger Dryas Boundary layers, not above it or below it. These discoveries provide strong evidence for a cosmic impact event at approximately 12,900 years ago that would have had enormous environmental consequences for plants, animals and humans across North America.”
The Clovis association is obviously interesting. One of the sediment layers in question was found adjacent to Clovis materials at Murray Springs, AZ. The Clovis culture seems to have lasted no more than 800 years, being replaced by more localized cultures apparently deriving from it. The question is whether we can attribute what may have been gradual cultural change to a sudden impact event and associated climate change.
Image: Nanometer-sized diamonds occur at the base a layer of sediment directly above the remains of extinct animals (mammoths, dire wolves, etc.) and artifacts from Clovis culture at the research site in Murray Springs, Arizona. Credit: University of Oregon.
So how convincing is the theory? I was interested to see that University of Hawaii scientist Gary Huss, who was a reviewer of the paper the team published today in Science, thinks the new work makes the theory more tenable. As quoted in the New York Times, Huss says:
“They have a hypothesis that explains several things that hard to explain any other way. Diamonds are less convincing by themselves, but they strengthen their case considerably.”
And geology consultant Allen West is quoted in the same story to the effect that the team has now traced microscopic diamond deposits to over thirty sites, ranging from offshore California to Germany. The six sites considered in the Science paper show the same pattern: The diamond layer correlates to the date of the supposed impact. Moreover, there’s this, also from the Times article:
…the scientists reported last month at a meeting of the American Geophysical Union in San Francisco, the carbon atoms inside some of the diamonds are lined up in a hexagonal crystal pattern instead of the usual cubic structure. The hexagonal diamonds, formed by extraordinary heat and pressure, have been found only at impact craters and within meteorites and cannot be formed in forest fires or volcanic eruptions, Dr. West said.
But where are the kinds of craters we would expect to find from such events? The lack of such points to how tentative this hypothesis is, but also to the need for continuing to explore it. For the timing of the would-be impact and the disappearance of ice age mammals amidst the sudden cooling makes for a puzzling series of coincidences, one that further field work may help us explain. With thirty sites in play, that could take a considerable amount of time.
The paper is Kennett et al., “Nanodiamonds in the Younger Dryas Boundary Sediment Layer,” Science Vol. 323, No. 5910 (2 January 2009), p. 94 (abstract). A University of Oregon news release is also available.
